[Python-Dev] RE: test_sort.py failure

[Tim Peters]

[Raymond Hettinger] wrote:
>>>> * list_ass_slice() should add a line to maintain them,
>>>> * list_resize() can drop the test for ob_item != NULL, and

[ditto]
> > Will put these in.

[Armin Rigo]
> I think the list_ass_slice() modification you are thinking about is obsolete
> since the check-in I documented in my latest e-mail.

Raymond did his checkin, and figured that out.  listobject.c is again
bug-free <wink>.

> list_resize() dropping the test for ob_item != NULL looks like a sane plan:
> the only C extensions that I can think of that could break would be the ones
> that for extreme performance hacking don't use PyList_SetSlice() to clear a
> list, but directly zero out ob_size and ob_item -- which is a real bad thing
> to do.

I'm not worried about that, but Raymond also added an assert() to
catch it if it happens.  Anyone writing such a gonzo extension must
have learned by now to develop in a debug build.

> About clearing lists efficiently: what about adding PyList_Clear() to the
> public API?

+1 provided the signature is changed to return void.  Even inside
listobject.c, the "it always returns 0" qurik is used only once.  It's
sometimes handy to have a function that always returns an error value
(e.g., PyErr_NoMemory), but I think an API function that always
returns a success value would be unprecedented.  That's unexpected,
hence jarring.

> It's clearer than the current
>   PyList_SetSlice(lst, 0, PyList_GET_SIZE(lst), (PyObject *)NULL)

Brrrrrrr.

> (which BTW could fail with a MemoryError up to yesterday;

Yet never has, and never would have <wink>.

> I bet not all users of this idiom were aware of that.

I'd be surprised if any were!.

> There was even an example of not checking the return value in listobject.c
> itself!)

Google quickly found two more, in an older revision of
itertoolsmodule.c.  There are probably more.  Looks like this
particular case ends up in list_clear() now, so that even users of the
broken "clear-all-and-don't-check idiom" would become immune from the
catastrophic consequences of their sloth.

> By contrast, PyList_Clear(), like list_clear(), would never fail and could return
> void.

Yup!